1. Field of the Invention
This invention relates to a liquid crystal element using a particular orientation film, and more particularly, to a twisted nematic liquid crystal element constituted of an electrode substrate, a dyed filter and the particular orientation film successively laminated in this order, or to a ferroelectric liquid crystal element.
2. Description of the Related Art
In a liquid crystal television panel using a conventional active matrix driving system, a thin film transistor (TFT) is connected to each picture element in a matrix state, a gate-on-pulse is applied to the TFT to conduct between the source and the drain, and at this time, and image signals are applied from the source and accumulated in a capacitor. The liquid crystal (for example, twisted nematic-TN liquid crystal) is driven corresponding to the accumulated image signals. Then, a color filter layer fitted to each picture element is subjected to optical switching to effect color display.
U.S. Pat. No. 4,367,924 to Clark and Lagerwall discloses a bistable ferroelectric liquid crystal (FLC) element. It has been tried to apply the ferroelectric liquid crystal element to a color display. Heretofore, in case of a liquid crystal element containing a color filter formed with a dyed layer at the electrode surface side, a dyeing medium and a material having a sufficient heat resistance for dyeing can not be selected. Therefore, the color filter is not thermally stable and the heat curing temperature at which an orientation controlling film is formed on the color filter is limited.
As a result, one method to overcome the above problem is that an orientation treatment such as a rubbing treatment is applied directly to the surface of the color filter so that the above-mentioned orientation controlling film is omitted. However, the inventors have found problems occur as shown below.
FIG. 1A shows a driving wave shape applied to picture elements of a ferroelectric liquid crystal.
FIG. 1B shows a voltage wave shape impressed to a liquid crystal itself at real time. That is, when a writing pulse, V.sub.ON, is applied to the ferroelectric liquid crystal from the opposing cross electrodes, the voltage wave shape substantially applied to the ferroelectric liquid crystal is as shown in FIG. 1B. That is, V.sub.0 upon applying a pulse is decrease by .DELTA.V.sub.0 at a rate of the time constant, .pi.=RC (R: resistance of FLC, C: capacitance of FLC). The less the resistance R, the larger the voltage drop, .DELTA.V.sub.0, and upon switching the pulse (upon trailing of pulse), -.DELTA.V.sub.0 of an opposite polarity is applied to the FLC. When this .vertline.-.DELTA.V.sub.0 .vertline. is larger than the reverse threshold voltage .vertline.-V.sub.th .vertline., for example, writing of black (opposite to writing of white) is effected. This is attributable to an electric field of an opposition direction (-.DELTA.V.sub.0) generated by discharge from capacitance of the dielectric member layer such as an orientation controlling film connected to the ferroelectric liquid crystal in series upon trailing of pulse.
Therefore, there is the following problem. As mentioned above, the liquid crystal element for color display contains color filters in the cell, and the dye in the color filter is dissolved in the ferroelectric liquid crystal. When such liquid crystal element is used for a long time, resistance R of the ferroelectric liquid crystal is lowered with the lapse of time until at last the value of the above-mentioned electric field of the opposite direction (-.DELTA.V.sub.0) exceeds the reverse threshold voltage resulting in the inoperativeness of the desired electro-optical switching.
When a row-subsequent writing system is applied to a ferroelectric liquid crystal element, there is, for example, a system wherein a pulse forming a first display state based on a first orientation state of a ferroelectric liquid crystal at phase t.sub.1 (becoming a first phase) is applied to all or some predetermined picture elements on row, and then a pulse capable of reversing a first display state to a second display state based on a second orientation state is applied to a picture element selected by phase t.sub.2 (which is to become the second phase).
According to this system, at phase t.sub.2, a pulse having a voltage less than the threshold voltage and of a polarity opposite to the pulse applied at phase t.sub.1 is applied to a picture element maintained at a first display state as shown in FIG. 2A.
In the case of row-subsequent writing system, it is necessary that the display state written at phase t.sub.1 is maintained without being reversed at phase t.sub.2. Therefore, aa voltage larger than the reverse threshold voltage is not to be applied at phase t.sub.2, however, as the result of the present inventors' study, the following problem has been found. That is, upon changing the pusle polarity from phase t.sub.1 to phase t.sub.2, a voltage of -(.alpha.V.sub.0 +.DELTA.V.sub.0) where a &lt;.vertline.V.sub.th .vertline./.vertline.V.sub.ON .vertline. and V.sub.th is a threshold voltage of the ferroelectric liquid crystal, is substantially applied to the liquid crystal layer as shown in FIG. 2B, and when -(.alpha.V.sub.0 +.DELTA.V.sub.0) is larger than the reverse threshold voltage, the picture element which is to maintain the first display state is reversed to the second display state at phase t.sub.2 resulting in that the desired display can not be formed.
When a ferroelectric liquid crystal element is prepared, it is practically difficult to form a monodomain showing bistability. Nonetheless, it has been demanded that an orientation controlling film of high performance be capable of not forming multidomain or single stable domain and, further, that the formation of monodomain showing bistability be free from orientation defect.
However, when an orientation controlling film for ferroelectric liquid crystal reported or published up to now is used, there is a tendency to form orientation defects and multidomain portions and single stable domain. portions coexist together with bistable domains.